The mammalian, and particularly the human, immune system is comprised of two arms, the innate and the adaptive immune systems. The cells of the innate immune system recognise, and respond to, infectious agents in a generic manner. Although the innate immune system is a vital immediate barrier to infection, it does not confer specific, long-lasting protection against foreign entities, such as invading pathogens. In contrast, the cells of the adaptive immune system recognise specific foreign entities, and induce immunological memory to these specific entities in the host.
DCs interact with components of the innate immune system soon after infection by a pathogen and also form a central part of the mammalian adaptive immune response. DCs differentiate from precursor cells into immature DCs. Immature DCs are present throughout the body and, although other cells of the immune system also participate in this role, they are the major cell type responsible for initiation of adaptive immune responses, primarily through their capacity to trigger T cell activation.
Immature DCs constantly sample their surrounding environment for infectious agents such as viruses, bacteria and parasites, through pattern recognition receptors (PRRs) such as toll-like receptors (TLRs) which recognise specific chemical signals on the foreign entity, e.g. on a pathogen's surface. Once an entity such as a pathogen has been identified as foreign, the immature DC internalises the entity or fragments of it and degrades the protein and lipid antigens into peptides and glycopeptides or lipid fragments which are presented on the DC surface.
In response to foreign entity recognition, and/or other signals within the cell's environment (e.g. inflammatory cytokines), the immature DC undergoes several changes collectively termed ‘maturation’ and starts to develop into a mature DC. The maturing DC up-regulates expression of major histocompatability complex (MHC), and MHC-related molecules such as CD1, which bind the foreign entity-derived peptides and glycopeptides, and lipids or glycolipids, respectively, and allow them to be displayed on the DC surface. Simultaneously, the DC up-regulates expression of cell surface receptors known as costimulatory molecules including CD80, CD86 and CD40, which act as co-receptors for T lymphocyte activation. In addition, the DC begins to migrate to lymphoid tissues such as the lymph nodes and/or spleen, following chemotactic signals. Once in the lymphoid tissues, the DC activates T lymphocytes, by presenting them with the peptides and glycopeptides or lipid fragments derived from the foreign entity and delivering the appropriate co-stimulatory signals. Such activated T lymphocytes are responsible for propagating the adaptive immune response. The foreign entity may be a pathogen, an allergen, a transplantation antigen such as a major transplant antigen or a minor transplant antigen, a blood group antigen or, in the case of an autoimmune response, a self-antigen incorrectly identified by the body as foreign.
As well as having a role in triggering T cell activation by antigen presentation and costimulation, mature DCs are involved in T cell regulation, such as polarisation of helper T cells into Th1, Th2, Th17 or regulatory (Treg) cells, the activation of cytotoxic T cells, and modulation of T cell homing, e.g. into the skin or gut and other mucosal sites.
The central role played by DCs in the adaptive immune response has led to interest in the modulation of DC function for therapeutic purposes, and there have been indications from animal models that DC modulators may be useful in the treatment of autoimmune and other inflammatory diseases (Subklewe et al, Human Immunology, 2007, 68(3), 147-155). It has also been suggested that DC modulators may be useful in the treatment of cancer (Banchereau, J. et al, Ann N Y Acad Sci, 2003, 987, 180-187 and Figdor, C. G. et al, Nature medicine, 2004, 10 (5), 475-480).
Ticks and some other haematophagous arthropods attach to their hosts, including mammals such as humans, and feed for extended periods of time. The components that haematophagous arthropods deliver to the hosts, including components in saliva, can potentially induce host immune responses. Such responses may be deleterious to the arthropods and therefore the arthropods may need to suppress them. Given the central role of DCs in triggering immunity, it may be advantageous to the arthropods to produce proteins that inhibit their function.
Haematophagous arthropods, and particularly ticks, may inhibit the host's immune system by inoculating the host with a variety of anti-inflammatory and immunomodulatory components (Ribeiro et al, Infectious Agents and Disease, 1992, 4(3), 143-152).
Several immunomodulatory molecules have been identified in tick saliva, including a homologue of macrophage migration inhibitory factor (MIF) (Jaworski et al, Insect Molecular Biology, 2001, 10(4), 323-331), a homologue of leukocyte elastase inhibitor which is secreted by human macrophages, monocytes and neutrophils (Leboulle et al, The Journal of Biological Chemistry, 2002, 277(12), 10083-10089), glycosylated protein p36, which is thought to suppress mitogen driven in vitro proliferation of murine spleen cells (Bergman et al, Journal of Parasitology, 2000, 86, 516-525), B cell inhibitory protein (BIP) (Hannier et al, Immunology, 2004, 113, 401-408), and B cell inhibitory factor (BIF) (Yu et al, Biochemical and Biophysical Research Communications, 2006, 343, 585-590).
Salp15 is a protein present in tick saliva which has been found to act on immature human DCs (Anguita et al, Immunity, 2002, 16, 849-859 and Hovius et al, Vector borne and Zoonotic diseases, 2007, 7(3), 296-302). However, assays involving the incubation of immature human DCs with Salp15 in the presence of an immune stimulus have shown that Salp15 does not inhibit the upregulation of costimulatory molecules (e.g. CD86). Salp15 does not therefore inhibit the maturation of human DCs.
Prostaglandin E2 (PGE2) is a non-protein molecule present in tick saliva that may modulate the activity of immature murine DCs, but has a minimal effect on maturation of these murine DCs (Sa-Nunes et al, The Journal of Immunology, 2007, 179, 1497-1505). PGE2 is capable of enhancing the maturation of human DCs but there is no evidence that it can act to inhibit the differentiation and maturation of human DCs.
It has also been suggested that tick saliva and salivary gland extract (SGE) may possess the ability to modulate the differentiation and maturation of murine DCs (Cavassani et al, Immunology, 2005, 114, 235-245, and Skallova et al, Journal of Immunology, 2008, 180, 6186-6192).
A DC modulatory molecule has been isolated from the tick Rhipicephalus appendiculatus, and shown to modulate the differentiation and maturation of mammalian DCs (PCT/GB2009/002219). In particular, this molecule has been shown to alter the development of differentiation cultures by inhibiting upregulation of CD1a and downregulation of CD14, a signature of differentiation into DCs, and to inhibit T cell proliferation. This molecule is known as Japanin (SEQ ID NO: 8) and a limited number of homologues of Japanin have also been identified in other tick species (SEQ ID NOs: 10, 12, 14 and 16). It would be advantageous to identify further homologues of Japanin which act as DC modulators for therapeutic purposes. It would also be advantageous to identify the key motifs and features of such DC modulatory proteins which are responsible for their function.